Efficient use of petroleum feedstock typically requires a refiner to convert relatively high molecular weight hydrocarbons to more valuable lower molecular weight gasoline range hydrocarbon materials. Catalytic cracking is one process used to produce the more valuable gasoline range materials.
Modern catalytic cracking processes typically react hydrocarbon vapors with a hot zeolitic cracking catalyst in a fluidized riser reactor. The cracking reaction proceeds as the catalyst and feedstock rise through the reactor, with a reaction mixture of predominantly spent catalyst and lower molecular weight hydrocarbons being discharged from the upper end of the reactor. After rising through the reactor, spent catalyst must be separated from the reaction mixture so that the cracked hydrocarbon products can be further processed and so that spent catalyst can be regenerated and reused.
In older "open" style catalyst disengagement systems, an initial solids separation typically is accomplished by causing a radical change in direction of reaction mixture flow. In such a system, the linear momentum of the catalyst particles forces the particles to impact on a surface near the point of flow redirection, thereby causing the particles to lose their momentum and fall from the mixture. At the same time, the relatively momentumless hydrocarbon vapors successfully negotiate the change in flow path direction and proceed through the system for further solids separation.
In these "open" systems, the solids-depleted gases are released into a large disengagement vessel which surrounds the riser reactor and contains one or more closed-bottomed cyclone separators, or "cyclones". The cyclones withdraw vapors from the vessel volume and cyclonically separate solids not removed in the initial disengagement step. After separating most of the solids from the withdrawn gas, the cyclones discharge a further solids-depleted gas along a closed vapor path leading out of the vessel.
At the same time that solids-depleted gas is discharged into the vessel, spent catalyst separated in the initial disengagement step accumulates in the bottom portion of the vessel as a dense bed of catalyst. The bed is stripped of entrained hydrocarbon vapors by passing stripping steam through the bed, thereby releasing a mixture of stripped vapors and stripping steam, or "stripping gas", into the vessel volume located above the dense bed. The stripping gas entering the vessel volume is drawn into the cyclones along with the solids-depleted gas from the initial separation step.
The "open" style system just described provides the additional advantage of damping pressure and catalyst surges known to occur in catalytic cracking riser reactors. Causes of these surges include equipment malfunctions and the sudden vaporization of water present in feedstock, as well as various unit pressure upsets. Because these riser surges are damped into the large vessel volume before the reaction products enter the secondary separation equipment, the surges do not degrade the separation efficiency of downstream devices as they otherwise would if not damped into the vessel volume. Examples of such "open" systems can be found in U.S. Pat. No. 4,500,423.
Unfortunately, the older "open" style system has been found to contribute to the undesired secondary thermal cracking of gasoline range materials when operated in the 1000 degree plus Fahrenheit temperature range common in modern catalytic cracking units. Because the cracked products mix with the large vessel volume before being withdrawn from the vessel by the secondary separation equipment, the cracked products can reside in the vessel long enough at high enough temperatures to significantly affect product yield. For example, estimates show that as much as ten percent of the desired gasoline range products can be lost if these products are exposed to temperatures of 1100.degree. F. for as little as 4 to 5 seconds.
To prevent undesired secondary thermal cracking, some refiners have turned to "closed" systems in which reaction products pass along a closed vapor path from a riser reactor to subsequent catalyst disengagement steps. By moving cracked vapors along a closed vapor path, the increased gas residence times caused by mixing cracked products into a large disengagement vessel volume is avoided.
While closed systems succeed in minimizing gas residence times and the associated undesired thermal cracking of reaction products, closed systems can suffer from an inability to mitigate the effects of pressure and catalyst surges. Specifically, because surges no longer vent into a large disengagement vessel volume, surges typically propagate into the cyclone, disturbing the cyclonic motion of materials inside the cyclone. This in turn reduces the cyclone's separation efficiency.
One method of dealing with unwanted surges in closed systems is to employ a mechanical solution such as the surge activated trickle valves disclosed in U.S. Pat. No. 4,581,205. This method permits surges to be vented into a large vessel volume, but is undesirable because it increases the mechanical complexity of the separation equipment and because it requires the continued operation of mechanical devices in the thermally severe and erosive catalytic cracking environment.
A more desirable solution to surge and secondary cracking problems is to employ an "open-bottomed" cyclone design as disclosed by Farnsworth in U.S. Pat. No. 4,478,708, the disclosure of which is hereby incorporated by reference. In this design, catalytically-cracked products and spent catalyst follow a closed vapor path into a cyclone having a bottom which opens into a relatively large disengagement vessel volume. Catalyst is cyclonically separated in the cyclone in much the same manner as in other cyclones well known in the art, but instead of falling into a dipleg, separated catalyst simply falls through the open cyclone bottom into the lower portion of the disengagement vessel for stripping and collection. Catalyst-depleted gas is withdrawn from the top of the cyclone and is passed through secondary separation cyclones as in many traditional closed-bottomed cyclone systems.
Farnsworth's design apparently succeeds because the lower pressure downstream of his open-bottomed cyclone causes the cyclone to appear to be a closed vapor path for gases even though the bottom of the cyclone is open. Only when cyclone inlet pressure increases significantly, such as under surge conditions, does the open bottom offer a vapor path into the large disengagement volume. Thus, Farnsworth's design represents an apparent improvement over the other designs already discussed.
While Farnsworth's open-bottomed cyclone design provides a partial solution to the surge and secondary cracking problems inherent in closed-vapor path catalytic cracking operations, his design suffers from a serious disadvantage that stems from the use of the open-bottomed cyclone. Specifically, while separated catalyst is falling downward toward the open bottom, stripping gas simultaneously must flow up into the cyclone's open bottom. The countercurrent flow of catalyst and vessel vapors can cause separated catalyst to become entrained in the stripping gas, thereby reducing the efficiency of the separator.
Until now, those skilled in the art have not recognized the problem of open-bottomed cyclone solids reentrainment. Instead, work to improve cyclone separators primarily has been directed toward improvements in the more traditional "closed-bottomed" cyclone designs. For example, Baillie, U.S. Pat. No. 4,081,249 teaches that closed-bottomed cyclone catalyst attrition can be reduced through the use of a collection of arresting vanes, flow reversing plates and baffles within the cyclone.
Other work by Baillie disclosed in U.S. Pat. No. 4,486,207 teaches that particle attrition can be reduced through the use of multiple cyclone inlets. The use of these multiple inlets permits increased cyclone throughput without increasing tangential wall velocity.
Parker, U.S. Pat. No. 4,455,220 discloses a combined cyclonic separation and stripping system in which a cyclonic separator is located directly over a stripping section and within a single closed vertical conduit. Parker employs a vortex stabilizer between the cyclonic and stripping sections to improve cyclone performance. It should be noted that Parker's design forces catalyst and stripped vapors to travel in a countercurrent manner between the stabilizer and the inner conduit wall housing the stabilizer, thereby also permitting entrainment of downwardly moving solids in the vapors moving upwardly from the stripping zone.
Kruse, allowed U.S. Ser. No. 07/529,204, also teaches the use of a cyclone separator located directly above a stripping zone within a single closed vertical conduit. Unlike Parker, Kruse employs a cone having an aperture at its apex to direct stripping gases along the longitudinal axis of his conduit. As with Parker, Kruse's invention essentially is a closed cyclone design intended for use outside of a disengagement vessel, and appears to have a region of countercurrently moving catalyst and stripping gas near the conduit wall.
None of the cyclone designs discussed above provide for a mechanically simple cyclone design which can accommodate pressure and catalyst surges while at the same time minimizing entrainment of downwardly moving catalyst in upwardly moving stripping gas entering the separator through the open cyclone bottom.